I hope I understand what you are asking.You don't need to press any key.Make sure that you are in Slide Options, with the layers list visible. All you need to do is to drag a picture from your thumbnails list on the layer that needs replacing.Just drag, and hold the mouse to the center of the layer that needs replacing, until it becomes marked (colored), and then release the mouse. The new picture will replace the old one.

You'll know that you are not doing it right, if instead of a colored layer, you are seeing a thick line, either at the top or at the bottom of the layer to be replaced.That line indicates the addition of a new layer, and not a replacement.